I'll give in my two cents:
Roxas' concept in Days basically requires Xion for anything to happen.
Exactly, Roxas' concept in
Days as it stands
presently.
Without some kind of outside influence, there's no conceivable way for either of them to move past "heartless automaton." Roxas would have patterned himself entirely on Axel, going through the motions and following orders because that's what he does. And, without a compassionate Roxas, Axel would have had no reason to change himself and begin acting for his own purposes instead of just doing whatever Saix says.
There was never truly any hint that Roxas had been in such a blank state previously when
358/2 Days didn't exist. I don't think Xion at all had that much influence in how Roxas broke from his "monotone" state. He appeared to progress slowly on his own and with each Disney world experience. Learning about love, friendship, etc., and
Axel there to guide him through all his curious questions. Xion being present or not present doesn't affect this growth. How can you say he wouldn't have been compassionate without her? When she wasn't a force or a friend, he was choked up that Axel might have died and was enjoying the time spent with a good friend. Feelings had sprung, and the reason for them were because of
Ven's Heart. I don't see how Roxas would have patterned himself around him either. He never seemed to follow what Axel did. He follows his advice, sure, but I never saw any clue that he would have been entirely like Axel if left entirely alone with him. That doesn't...equate. Especially since Roxas questioned the Organization's motives before shit had started.
And "but Roxas would have gotten Sora's memories if Xion wasn't there" doesn't work either, because there's no reason to think Roxas would have actually gotten those memories without her.
The basic process that's been said in-game
and interviews is that
Roxas got Sora's memories, but Xion sucked them from him to itself. So, in general terms, if Xion wasn't there, what vessel would the memories remain in?
CoM made it possible for Sora's old memories to be a mess and broken - it isn't farfetched to believe those pieces would have transported to the
other half of himself, especially since Nobodies are suppose to have their original self's memories.
Now, you can say "Roxas was only given that personality because they needed Xion to exist," but that's not necessarily the case. Nomura did say that, from the beginning, he thought Roxas needed the influence of a girl his age to get him to leave, even if Xion herself wasn't created to fill that spot until later.
And I don't believe that for a second. Every time he was asked why Roxas left, it had been because of Sora's and/or Axel's actions/lies. Like goldpanner presented.
Never did he once say before
Days was made into a concept that there was always another outside influence or person. I find that odd, considering he had no problem revealing anything else on the matter. And even in the game now, you can see that, as much as him or the staff attempted to shoe-horn Xion into everything they could that didn't even need an explanation to make her somewhat relevant, they couldn't change what was established in
KH2, so Roxas stayed unmentioning Xion to Axel as his reason for abandoning them.
That, and personally, the idea of a girl of all things making a guy see that an already shady Organization aren't trustworthy is rather ridiculous, cliche, and utterly stupid when he didn't even need her to figure that out - enough to ask Axel about their goals at some point earlier on in the same game.
